1. Field of the Invention
The present invention relates generally to a system and method for allocating a Channel Quality Information Channel (CQICH) in a communication system.
2. Description of the Related Art
Active research on the next generation communication system is being conducted to provide users with desired services which can meet a desired Quality-of-Service (QoS) at a high data rate.
A wireless Local Area Network (LAN) communication system and a wireless Metropolitan Area Network (MAN) communication system support a high data rate. The wireless MAN communication system, one of the typical Broadband Wireless Access (BWA) communication systems, has broader service coverage and supports a higher data rate, compared with the wireless LAN communication system. Therefore, active research on the next generation communication system is being conducted to develop a new communication system that guarantees mobility of a mobile station (MS) and QoS to the wireless LAN communication system and the wireless MAN communication system, both of which guarantee a higher data rate, to thereby support a higher data rate of the next generation communication system.
A system employing an Orthogonal Frequency Division Multiplexing (OFDM) scheme and/or an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support a broadband transmission network for physical channels of the wireless MAN communication system is known as an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system. The IEEE 802.16 communication system, applies the OFDM/OFDMA scheme to the wireless MAN communication system, and can therefore support high-speed data transmission by transmitting physical channel signals using a plurality of subcarriers. For convenience, the following description will be made with reference to the IEEE 802.16 communication system, one of the typical BWA communication systems.
A description will now be made of a Fast Base Station Switching (FBSS) scheme among various handover schemes for connection switching from a serving base station (BS) to a target BS in the IEEE 802.16 communication system.
The FBSS scheme is a handover scheme that uses selection diversity and fast switching mechanism to improve link quality. In the FBSS scheme, a mobile station (MS) exchanges signals with only one active BS, i.e., an anchor BS, among active BSs included in the MS's diversity set. The anchor BS is a type of serving BS, and is replaceable (or switchable) according to a BS selection scheme of the MS. A description will now be made of the diversity set.
Each of active BSs constituting the MS's diversity set is distinguished with a temporary BS indicator (Temp BS ID), and all of the active BSs constituting the diversity set share capability, security parameters, service parameters, and full Medium Access Control (MAC) context information of the corresponding MS. Each of the active BSs constituting the diversity set can be excluded from the diversity set or a new BS currently not included in the diversity set can be added to the diversity set as a new active BS according to strength of a signal received (by the MS) from each active BS.
The MS selects one of the active BSs constituting the diversity set as an anchor BS. In this case, strength, for example, carrier-to-interference and noise ratio (CINR), of a signal received from the active BS selected as the anchor BS is highest, compared with CINRs of signals transmitted by the other active BSs constituting the diversity set. Therefore, in the FBSS scheme, the MS “performs communication” with only the anchor BS. The anchor BS that currently communicates with the MS will be referred to as a “serving anchor BS.” The MS measures signal quality for each of the active BSs constituting the diversity set while communicating with the serving anchor BS. Measuring the signal quality means measuring a CINR of the corresponding signal. Upon detecting the presence of any active BS providing higher signal quality than that of the current serving anchor BS as a result of the signal quality measurement, the MS determines to handover from the serving anchor BS to the active BS that provides the higher signal quality than that of the current serving anchor BS, accomplishing fast switching. Herein, an active BS, to which the MS performs fast switching, that provides a higher signal quality than that of the current serving anchor BS will be referred to as a “target anchor BS.”
With reference to FIG. 1, a description of an FBSS-based fast switching operation of an MS in a conventional IEEE 802.16 communication system will now be given. FIG. 1 is a flowchart illustrating an FBSS-based fast switching operation of an MS in a conventional IEEE 802.16 communication system.
Referring to FIG. 1, an MS performs communication with a serving anchor BS in step 101. Thereafter, the MS measures signal quality for each active BSs constituting the MS's diversity set in step 103. Herein, the phrase an MS performs communication with a serving anchor BS means that allocation information for the MS is included in MAP messages, i.e., a downlink MAP (DL-MAP) message and an uplink MAP (UL-MAP) message, transmitted by the serving anchor BS.
The MS determines in step 105 whether it should switch the serving anchor BS, i.e., it should perform a fast switching operation. Herein, the MS determines to switch the serving anchor BS if there is any active BS having a higher signal quality than that of the serving anchor BS. If there is no need to switch the anchor BS as a result of the determination, the MS returns to step 101. However, if it is determined in step 105 that there is a need to switch the anchor BS, the MS proceeds to step 107.
In step 107, the MS detects a start point of a switch period. Herein, the switch period refers to a period in which an MS reports a serving anchor BS that it will perform a fast switching operation to a new active BS, i.e., a target anchor BS, rather than the serving anchor BS. The switch period will be described later in detail with reference to FIG. 2. In step 109, after detecting the start point of the switch period, the MS transmits a Temp BS ID of a target anchor BS to which it will perform a fast switching operation from the serving anchor BS, to the serving anchor BS in the switch period. Herein, the MS transmits the Temp BS ID of the target anchor BS (i.e., a codeword corresponding to the Temp BS ID), to the serving anchor BS through a CQICH allocated from the serving anchor BS. The CQICH is a channel used by the MS to transmit a control signal in a process of feeding back a Channel quality information (CQI) value of the serving anchor BS or performing an FBSS-based fast switching operation.
Possible codewords mapped to corresponding Temp BS IDs are shown in Table 1 below.
TABLE 1ValueDescription0b100000Active BS for TEMP_BS_ID = 0000b100001Active BS for TEMP_BS_ID = 0010b100010Active BS for TEMP_BS_ID = 0100b100011Active BS for TEMP_BS_ID = 0110b100100Active BS for TEMP_BS_ID = 1000b100101Active BS for TEMP_BS_ID = 1010b100110Active BS for TEMP_BS_ID = 1100b100111Active BS for TEMP_BS_ID = 1110b101000Acknowledgement of Receipt of Anchor BS Switch IE
In Table 1, “Value” represents codewords mapped to corresponding Temp BS IDs of active BSs. For example, if the MS desires to perform fast switching from a serving anchor BS to a target anchor BS with Temp BS ID=“011”, the MS transmits a codeword “0b100011” mapped to the Temp BS ID=‘011’ to the serving anchor BS through a CQICH. In Table 1, a codeword ‘0b101000’ is not a codeword mapped to a Temp BS ID of an active BS, but is a codeword used by the serving anchor BS to inform the MS that it has received a transmitted anchor BS switch Information Element (Anchor BS Switch IE). A format of the Anchor BS Switch IE will be described later in detail.
In step 111, the MS transmits a CQI value for the serving anchor BS through a CQICH being identical to the CQICH used for transmitting a codeword mapped to the Temp BS ID of the target anchor BS. The MS determines in step 113 whether an Anchor BS Switch IE indicating a fast switching cancel request to the target anchor BS is received from the serving anchor BS. A format of the Anchor BS Switch IE is shown in Table 2.
TABLE 2SyntaxSizeNotesAnchor BS Switch IE( ){N_Anchor BS_Switch4 bitsNumber of Anchor BS switching for(i=0;i<N_Anchorindicated in this IEBS_Switch;i++){ CID16 bits Basic CID of a MS whose anchor BS switching is indicated in this IE Action code2 bits00 - The MS shall switch to the Anchor BS specified in the fast Anchor BS selection information in the Fast-feedback channel, at the default time specified by the switching period defined in the DCD01 - The MS shall switch to the Anchor BS specified in this IE and at the action time specified in this IE10 - The MS shall cancel all anchor switching procedure, stop switching timer and remain on the current anchor BS11 - reserved if (Action code ==01){  Action time(A)3 bitsIn units of frames, 000 means the MS shall switch at the default time specified by the switching period defined in DCD  TEMP BS ID3 bitsTEMP BS ID of the anchor BS to  }switch to. If(Action code==00 ||Action code==01){ CQICH Allocation1 bitTo indicate if CQICH allocation at Indicatorthe new Anchor BS is included in this IE if(CQICH AllocationIndicator ==1){  CQICH IDVariableIndex to uniquely identify the CQICH resource assigned to the MS after the MS switched to the new Anchor BS  Feedback channel6 bitsIndex to the Fast feedback channel offsetregion of the new Anchor BS marked by UIUC = 0  Period(p)2 bitsA CQI feedback is transmitted on the CQICH every 2p frames  Frame offset3 bitsThe MS starts reporting at the frame of which the number has the same 3 LSB as the specified frame offset. If the current frame is specified, the MS should start reporting in 8 frames.  Duration(d)3 bitsA CQI feedback is transmitted on the CQI channels indexed by the CQICH ID for framesif d = 0, the CQICH is deallocatedif d = 111, the MS should report until the BS commend for the MS to stop  } }}
In Table 2, N_Anchor BS_Switch represents the number of anchor BS fast switching informations included in the Anchor BS Switch IE, and the Anchor BS Switch IE can include anchor BS fast switching informations allocated to a plurality of MSs because it is included in a MAP message before being broadcasted. Herein, the number of anchor BS fast switching informations is equivalent to “N_Anchor BS_Switch”, and each of “anchor BS fast switching informations” is equivalent to each of informations in “for (i=0; i<N_Anchor BS_Switch; i++)” structure. The Anchor BS Switch IE includes a Connection Identifier (CID) for each of the anchor BS fast switching informations, an Action code, and CQICH allocation information for the Action code. The CID represents a basic CID for each MS, and the Action code represents information on a switch period in which the MS fast-switches to the target anchor BS, or information used for commanding the MS to fast-switch to a target anchor BS indicated by the serving anchor BS in a period (i.e., a switch period), indicated by the serving anchor BS, or for commending the MS to cancel the fast switching.
As shown in Table 2, Action code=“00” indicates that the MS should perform a fast switching operation to the target anchor BS at a default time in a switch period defined in a Downlink Channel Descriptor (DCD) message, Action code=‘01’ indicates that the MS should perform a fast switching operation to the target anchor BS at an action time specified in the Anchor BS Switch IE, and Action code=‘10’ indicates that the MS should cancel the fast switching operation and maintain communication with the serving anchor BS.
In Table 2, CQICH Allocation Indicator represents CQICH allocation information to be used by the MS in the target anchor BS. The CQICH allocation information includes CQICH ID, Feedback channel offset indicating a position of a CQICH actually allocated to the MS in a CQI field defined in a UL-MAP message, Period (p) indicating a period in which the MS reports a CQI value, Frame offset indicating a time at which the MS starts reporting a CQI value, and Duration (d) indicating information on effective duration of a CQICH allocated from the target anchor BS. Therefore, an MS, receiving the CQICH allocation information, reports a CQI value to the target anchor BS through the CQICH every 2p frame for frame duration after the Frame offset time.
If it is determined in step 113 that an Anchor BS Switch IE indicating a fast switching cancel request to the target anchor BS is received from the serving anchor BS (i.e., the Action code is set to “10”), the MS proceeds to step 115. In step 115, the MS cancels a fast switching operation (from the serving anchor BS to the target anchor BS), and then ends the process.
However, if it is determined in step 113 that an Anchor BS Switch IE indicating a fast switching cancel request to the target anchor BS is not received from the serving anchor BS, the MS proceeds to step 117 where it determines whether the switch period has expired. If it is determined that the switch period has not expired, the MS returns to step 109 where it can repeatedly transmit information indicating a desired fast switching operation to the target anchor BS (i.e., the MS transmits a codeword mapped to a Temp BS ID of the target anchor BS), to the serving anchor BS. The repeated transmission of the codeword mapped to the Temp BS ID of the target anchor BS continues until the switch period has expired in step 117 or the Anchor BS Switch IE indicating a fast switching cancel request to the target anchor BS is received from the serving anchor BS in step 113 even though the switch period has not expired.
If it is determined in step 117 that the switch period has expired, the MS proceeds to step 119 where it determines whether CQICH allocation information to be used in the target anchor BS has been received from the serving anchor BS. Herein, the MS can be aware of the CQICH allocation information to be used in the target anchor BS through CQICH allocation information previously allocated before the MS performs a fast switching operation from the serving anchor BS to the target anchor BS, or through Anchor BS Switch IE shown in Table 2 for the switch period. Alternatively, even in the process of adding a new active BS to it's the MS's diversity set, the MS can acquire CQICH allocation information of the active BS through a handover control message. The operation of managing the diversity set through the handover control message will not be further described herein because it is not directly related to the present invention.
If it is determined in step 119 that the CQICH allocation information to be used in the target anchor BS has not been received, the MS proceeds to step 121 where it performs a fast switching operation from the serving anchor BS to the target anchor BS and then waits a preset time. Further, the MS determines whether CQICH allocation information is received through a MAP message broadcasted by the target anchor BS within the preset time. A format of CQICH allocation information, i.e., CQICH Alloc IE, in the MAP message including the CQICH allocation information is shown in Table 3 below.
TABLE 3SyntaxSizeNotesCQICH Alloc IE  CQICH IDVariableIndex to uniquely identify the CQICH resource assigned to the MSThe size of this field is dependent on system parameter defined in DCD  Feedback6 bitsIndex to the Fast feedback channel region channel offsetmarked by UIUC = 0  Period(p)2 bitsA CQI feedback is transmitted on the CQICH every 2p frames  Frame offset3 bitsThe MS starts reporting at the frame of which the number has the same 3 LSB as the specified frame offset. If the current frame is specified, the MS should start reporting in 8frames.  Duration(d)3 bitsA CQI feedback is transmitted on the CQI channels indexed by the CQICH ID for frames if d = 0, the CQICH is deallocatedif d = 111, the MS should report until the BS commend for the MS to stop } }}
In Table 3, CQICH Alloc IE allocated in a target anchor BS indicates CQICH allocation information used by an MS to exchange control signals with the target anchor BS, and the CQICH Alloc IE includes CQICH ID, Feedback channel offset for providing actual location information of a CQICH in a CQI field defined in a UL-MAP message, and Frame offset indicating a time at which the MS actually starts reporting a CQI value after being allocated a CQICH. In addition, the CQICH Alloc IE includes Period (p) indicating a period in which the MS reports the CQI value, and Duration (d) indicating an effective time for which the MS can use the allocated CQICH.
If it is determined in step 121 that the CQICH allocation information is not received, the MS proceeds to step 123. In step 123, the MS performs a network re-entry operation with the target anchor BS and then ends the process because it failed to receive CQICH allocation information from the target anchor BS, i.e., because the MS failed to be allocated the CQICH from the target anchor BS even after the MS completed a fast switching operation from the serving anchor BS to the target anchor BS. Herein, the term “network re-entry operation” refers to an operation of acquiring synchronization with the target anchor BS and then performing initial ranging, registration and authentication.
However, if it is determined in step 119 that the CQICH allocation information to be used in the target anchor BS has been received, the MS proceeds to step 125. Also, if it is determined in step 121 that the CQICH allocation information has been received, the MS proceeds to step 125. In step 125, the MS performs a fast switching operation and communication with the target anchor BS through the CQICH allocated thereto, and then ends the process.
The FBSS-based fast switching operation of an MS in the conventional IEEE 802.16 communication system has been described so far with reference to FIG. 1. Next, with reference to FIG. 2, a description will be made of a format of an Anchor Switch Reporting (ASR) slot used in an FBSS-based fast switching operation of an MS in the general IEEE 802.16 communication system.
FIG. 2 is a diagram illustrating a format of an ASR slot used in an FBSS-based fast switching operation of an MS in a conventional IEEE 802.16 communication system. Reference numeral 201 represents transmission of a control signal from an MS to a serving anchor BS, shown on the time axis, reference numeral 203 represents transmission of a control signal from the MS and a target anchor BS, also shown on the time axis. As illustrated in FIG. 2, in the FBSS scheme, the time axis is divided in ASR slots. Each of the ASR slots is comprised of a plurality of, for example, M frames. The number, M, of frames constituting the ASR slot is determined by a BS, and the MS obtains the number, M, of frames constituting the ASR, determined by the BS, through a DCD message.
Reference numeral 205 represents an ASR slot before a fast switching operation performed with the serving anchor BS by the MS, and reference numerals 207 and 209 represent ASR slots in a switch period, for which the MS transmits a control signal used for informing the serving anchor BS of the fast switching operation from the serving anchor BS to the target anchor BS. The switch period includes one or more ASR slots. The number of ASR sots constituting the switch period is also determined by the BS, and the MS finds the number of ASR slots constituting the time switch, determined by the BS, through the DCD message. Reference numeral 211 represents an ASR slot in which the MS performs communication with the target anchor BS after performing a fast switching operation from the serving anchor BS to the target anchor BS, i.e., after completing the switching operation.
In the ASR slot 205 before a fast switching operation, in which the MS is communicating with the serving anchor BS, the MS transmits a CQI value of the serving anchor BS to the serving anchor BS every frame in the ASR slot 205 while performing communication with the serving anchor BS. In the ASR slot 205, if the MS determines that it should a perform fast switching operation from the serving anchor BS to the target anchor BS, the MS waits until termination of the corresponding ASR slot, i.e., the ASR slot 205, where it determined to perform the fast switching operation, instead of immediately performing the fast switching operation at the time where it determined to perform the fast switching.
After termination of the ASR slot 205, the MS informs the serving anchor BS for the switch period that it will perform fast switching from the serving anchor BS to the target anchor BS. In this case, the MS transmits, to the serving anchor BS, a CQI value CA of the serving anchor BS and an indicator IB indicating that it will perform a fast switching operation from the serving anchor BS to the target anchor BS. Although the indicator IB and the CQI value CA are alternately transmitted in FIG. 2 on a frame-by-frame basis, by way of example, a pattern for transmitting the indicators IB and the CQI values CA in the switch period can be variably determined by the MS. At any rate, the MS should transmit the indicator IB and the CQI value CA at least once in a first ASR slot of the switch period.
After the end of the switch period, the MS that performed the fast switching operation from the serving anchor BS to the target anchor BS, transmits a CQI value of the target anchor BS in the ASR slot 211 while performing communication with the target anchor BS.
As described above, in the FBSS scheme, the MS performs a fast switching operation from the serving anchor BS to the target anchor BS by transmitting control signals using a CQICH. However, if the MS fails to be previously allocated a CQICH from the target anchor BS after performing the fast switching operation from the serving anchor BS to the target anchor BS in the switch period, the MS should perform network re-entry with the target anchor BS as described above, causing a delay in connection setup.
If the MS fails to acquire CQICH allocation information from the target anchor BS even after performing the FBSS-based fast switching operation, the MS performs a network re-entry operation with the target anchor BS, causing a connection setup delay. Accordingly, there is a need for a scheme capable of enabling the MS to rapidly set up a connection with the target anchor BS after performing the FBSS-fast switching operation.